Visible light responsive Bi2S3-graphene/TiO2 nanocomposites: one pot-hydrothermal synthesize and improved photocatalytic properties

Abstract

Herein, we obtained chemically bonded Bi₂S₃-Graphene/TiO₂ composites using a facile one pot-hydrothermal method. During the hydrothermal reaction, both of the reduction of graphene oxide to graphene and loading of Bi₂S₃ and TiO₂ particles on graphene nanosheet were achieved. The resulting composites are characterized by X-Ray diffraction (XRD), Raman spectroscopy and Transmission Electron Microscopy (TEM). The optical properties are studied using UV–visible diffuse reflectance spectroscopy (DRS), which confirms that the spectral responses of the composite catalysts are extended to the visible light region. The dramatically enhanced activity of Bi₂S₃-Graphene/TiO₂ composite photocatalysts can be attributed to great adsorptivity of dyes, extended light absorption range, and efficient charge separation properties, simultaneously. This work may provide new insights into the design of novel composite photocatalysts system with efficient visible light activity.     

One‐Step Transfer and Integration of Multifunctionality in CVD Graphene by TiO2/Graphene Oxide Hybrid Layer

We present a straightforward method for simultaneously enhancing the electrical conductivity, environmental stability, and photocatalytic properties of graphene films through one‐step transfer of CVD graphene and integration by introducing TiO₂/graphene oxide layer. A highly durable and flexible TiO₂ layer is successfully used as a supporting layer for graphene transfer instead of the commonly used PMMA. Transferred graphene/TiO₂ film is directly used for measuring the carrier transport and optoelectronic properties without an extra TiO₂ removal and following deposition steps for multifunctional integration into devices because the thin TiO₂ layer is optically transparent and electrically semiconducting. Moreover, the TiO₂ layer induces charge screening by electrostatically interacting with the residual oxygen moieties on graphene, which are charge scattering centers, resulting in a reduced current hysteresis. Adsorption of water and other chemical molecules onto the graphene surface is also prevented by the passivating TiO₂ layer, resulting in the long term environmental stability of the graphene under high temperature and humidity. In addition, the graphene/TiO₂ film shows effectively enhanced photocatalytic properties because of the increase in the transport efficiency of the photogenerated electrons due to the decrease in the injection barrier formed at the interface between the F‐doped tin oxide and TiO₂ layers.     

Metal–Organic‐Framework‐Based Catalysts for Photoreduction of CO2

Photoreduction of CO₂ into reusable carbon forms is considered as a promising approach to address the crisis of energy from fossil fuels and reduce excessive CO₂ emission. Recently, metal–organic frameworks (MOFs) have attracted much attention as CO₂ photoreduction‐related catalysts, owing to their unique electronic band structures, excellent CO₂ adsorption capacities, and tailorable light‐absorption abilities. Recent advances on the design, synthesis, and CO₂ reduction applications of MOF‐based photocatalysts are discussed here, beginning with the introduction of the characteristics of high‐efficiency photocatalysts and structural advantages of MOFs. The roles of MOFs in CO₂ photoreduction systems as photocatalysts, photocatalytic hosts, and cocatalysts are analyzed. Detailed discussions focus on two constituents of pure MOFs (metal clusters such as Ti–O, Zr–O, and Fe–O clusters and functional organic linkers such as amino‐modified, photosensitizer‐functionalized, and electron‐rich conjugated linkers) and three types of MOF‐based composites (metal–MOF, semiconductor–MOF, and photosensitizer–MOF composites). The constituents, CO₂ adsorption capacities, absorption edges, and photocatalytic activities of these photocatalysts are highlighted to provide fundamental guidance to rational design of efficient MOF‐based photocatalyst materials for CO₂ reduction. A perspective of future research directions, critical challenges to be met, and potential solutions in this research field concludes the discussion.     

In Situ Irradiated X‐Ray Photoelectron Spectroscopy Investigation on a Direct Z‐Scheme TiO2/CdS Composite Film Photocatalyst

Inspired by nature, artificial photosynthesis through the construction of direct Z‐scheme photocatalysts is extensively studied for sustainable solar fuel production due to the effectiveness in enhancing photoconversion efficiency. However, there is still a lack of thorough understanding and direct evidence for the direct Z‐scheme charge transfer in these photocatalysts. Herein, a recyclable direct Z‐scheme composite film composed of titanium dioxide and cadmium sulfide (TiO₂/CdS) is prepared for high‐efficiency photocatalytic carbon dioxide (CO₂) reduction. In situ irradiated X‐ray photoelectron spectroscopy (ISI‐XPS) confirms the direct Z‐scheme charge‐carrier migration pathway in the photocatalytic system. Furthermore, density functional theory simulation identifies the intrinsic cause for the formation of the direct Z‐scheme heterojunction between the TiO₂ and the CdS. Thanks to the significantly enhanced redox abilities of the charge carriers in the direct Z‐scheme system, the photocatalytic CO₂ reduction performance of the optimized TiO₂/CdS is 3.5, 5.4, and 6.3 times higher than that of CdS, TiO₂, and commercial TiO₂ (P25), respectively, in terms of methane production. This work is a valuable guideline in preparation of highly efficient recyclable nanocomposite for photoconversion applications.     

Structure,Synthesis, and Applications of TiO2 Nanobelts

TiO₂ semiconductor nanobelts have unique structural and functional properties, which lead to great potential in many fields, including photovoltaics, photocatalysis, energy storage, gas sensors, biosensors, and even biomaterials. A review of synthetic methods, properties, surface modification, and applications of TiO₂ nanobelts is presented here. The structural features and basic properties of TiO₂ nanobelts are systematically discussed, with the many applications of TiO₂ nanobelts in the fields of photocatalysis, solar cells, gas sensors, biosensors, and lithium‐ion batteries then introduced. Research efforts that aim to overcome the intrinsic drawbacks of TiO₂ nanobelts are also highlighted. These efforts are focused on the rational design and modification of TiO₂ nanobelts by doping with heteroatoms and/or forming surface heterostructures, to improve their desirable properties. Subsequently, the various types of surface heterostructures obtained by coupling TiO₂ nanobelts with metal and metal oxide nanoparticles, chalcogenides, and conducting polymers are described. Further, the charge separation and electron transfer at the interfaces of these heterostructures are also discussed. These properties are related to improved sensitivity and selectivity for specific gases and biomolecules, as well as enhanced UV and visible light photocatalytic properties. The progress in developments of near‐infrared‐active photocatalysts based on TiO₂ nanobelts is also highlighted. Finally, an outline of important directions of future research into the synthesis, modification, and applications of this unique material is given.     

Reduced graphene oxide and CdTe nanoparticles co-decorated TiO2 nanotube array as a visible light photocatalyst

TiO₂ nanotube array (TiO₂ NT) was co-decorated by reduced graphene oxide (RGO) and CdTe nanoparticles (NPs) through a simple one-step electrodeposition process. RGO film was formed on the top surface of TiO₂ NT and CdTe NPs homogeneously dispersed within the RGO sheets and on the inner/outer walls of TiO₂ NT. Resulting from the synergetic effect of RGO and CdTe, the photocatalytic activity of the ternary RGO/CdTe–TiO₂ NT photocatalyst far exceeded those of bare TiO₂ NT, RGO-TiO₂ NT, and CdTe–TiO₂ NT photocatalysts in the degradation of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) under simulated solar light or visible light irradiation. After 180-min UV–Vis (or visible light) irradiation, almost 100 % (or 96 %) 2,4-D removal efficiency was achieved on RGO/CdTe–TiO₂ NT, much higher than 42 % (or 2 %) on bare TiO₂ NT, 58 % (or 10 %) on RGO–TiO₂ NT, and 52 % (or 41 %) on CdTe–TiO₂ NT. This study will inspire better design of advanced photocatalysts with high visible-light photocatalytic activity.     

One‐Dimensional Densely Aligned Perovskite‐Decorated Semiconductor Heterojunctions with Enhanced Photocatalytic Activity

By using one‐dimensional rutile TiO₂ nanorod arrays as the structure‐directing scaffold as well as the TiO₂ source to two consecutive hydrothermal reactions, densely aligned SrTiO₃‐modified rutile TiO₂ heterojunction photocatalysts are crafted for the first time. The first hydrothermal processing yielded nanostructured rutile TiO₂ with the hollow openings on the top of nanorods (i.e., partially etched rutile TiO₂ nanorod arrays; denoted PE‐TNRAs). The subsequent second hydrothermal treatment in the presence of Sr²⁺ transforms the surface of partially etched rutile TiO₂ nanorods into SrTiO₃ nanoparticles via the concurrent dissolution of TiO₂ and precipitation of SrTiO₃ while retaining the cylindrical shape (i.e., forming SrTiO₃‐decorated rutile TiO₂ composite nanorods; denoted STO‐TNRAs). The structural and composition characterizations substantiate the success in achieving STO‐TNRA nanostructures. In comparison to PE‐TNRAs, STO‐TNRA photocatalysts exhibit higher photocurrents and larger photocatalytic degradation rates of methylene blue (3.21 times over PE‐TNRAs) under UV light illumination as a direct consequence of improved charge carrier mobility and enhanced electron/hole separation. Such 1D perovskite‐decorated semiconductor nanoarrays are very attractive for optoelectronic applications in photovoltaics, photocatalytic hydrogen production, among other areas.     

Ordered Single‐Crystalline Anatase TiO2 Nanorod Clusters Planted on Graphene for Fast Charge Transfer in Photoelectrochemical Solar Cells

Achieving efficient charge transport is a great challenge in nanostructured TiO₂‐electrode‐based photoelectrochemical cells. Inspired by excellent directional charge transport and the well‐known electroconductibility of 1D anatase TiO₂ nanostructured materials and graphene, respectively, planting ordered, single‐crystalline anatase TiO₂ nanorod clusters on graphene sheets (rGO/ATRCs) via a facial one‐pot solvothermal method is reported. The hierarchical rGO/ATRCs nanostructure can serve as an efficient light‐harvesting electrode for dye‐sensitized solar cells. In addition, the obtained high‐crystallinity anatase TiO₂ nanorods in rGO/ATRCs possess a lower density of trap states, thus facilitating diffusion‐driven charge transport and suppressing electron recombination. Moreover, the novel architecture significantly enhances the trap‐free charge diffusion coefficient, which contributes to superior electron mobility properties. By virtue of more efficient charge transport and higher energy conversion efficiency, the rGO/ATRCs developed in this work show significant advantages over conventional rGO–TiO₂ nanoparticle counterparts in photoelectrochemical cells.     

Amine Functionalized Metal–Organic Framework Coordinated with Transition Metal Ions: d–d Transition Enhanced Optical Absorption and Role of Transition Metal Sites on Solar Light Driven H2 Production

Design and development of efficient photocatalysts for H₂ production from water and sunlight have gained significant attention as the solar assisted approach is considered to be a promising approach for the generation of clean fuel. However, the poor charge carrier separation and light harvesting ability of existing photocatalysts limits the efficiency of photoconversion of water. In this work, the synthesis of transition metal ions (M²⁺ = Co²⁺, Cu²⁺, and Ni²⁺) coordinated with Ti‐metal organic frameworks (Ti‐MOFs) through a simple post‐synthetic coordination method for efficient solar light‐driven H₂ production is reported. Notably, coordination of M²⁺ ions with Ti‐MOF significantly improves the optical absorption by d–d transitions and the multimetal sites facilitate the fast charge carrier separation, thereby enhancing the solar light‐driven H₂ production activity. Very interestingly, the rate of solar light‐driven H₂ production is varied with respect to different metal ions coordination due to the position of d–d bands absorption in the solar spectrum, and the complexing tendency of M²⁺ ions with sacrificial electron donors. A maximum solar H₂ production rate of 1583.55 µmol h^?1 g^?1 is achieved with a Cu²⁺ coordinated Ti‐MOF, which is ≈13 fold higher than that of the pristine Ti‐MOF.     

3D Aerogel of Graphitic Carbon Nitride Modified with Perylene Imide and Graphene Oxide for Highly Efficient Nitric Oxide Removal under Visible Light

3D materials are considered promising for photocatalytic applications in air purification because of their large surface areas, controllability, and recyclability. Here, a series of aerogels consisting of graphitic‐carbon nitride (g‐C₃N₄) modified with a perylene imide (PI) and graphene oxide (GO) are prepared for nitric oxide (NO) removal under visible‐light irradiation. All of the photocatalysts exhibit excellent activity in NO removal because of the strong light absorption and good planarity of PI–g‐C₃N₄ coupled with the favorable charge transport properties of GO, which slow the recombination of electron–hole pairs. The aerogel containing thiophene displays the most efficient NO removal of the aerogel series, with a removal ratio of up to 66%. Density functional theory calculations are conducted to explain this result and recycling experiments are carried out to verify the stability and recyclability of these photocatalysts.     

Reduced graphene oxide-based photocatalysts containing Ag nanoparticles on a TiO2 nanotube array

A simple one-step electrochemical deposition method was demonstrated to fabricate reduced graphene oxide/Ag nanoparticle co-decorated TiO₂ nanotube arrays (RGO/Ag–TiO₂NTs) photocatalyst in this study. The structures and properties of these photocatalysts were characterized using scanning electron microscope, X-ray diffraction, UV–Vis diffuse reflection spectra, and photoluminescence. By taking the advantages of TiO₂, graphene, and Ag nanoparticles (AgNPs), RGO/Ag–TiO₂NTs showed a greatly improved photocatalytic activity compared with the bare TiO₂NTs, Ag–TiO₂NTs or RGO–TiO₂NTs. The deposited RGO and AgNPs not only reduce the recombination of photogenerated electrons and holes, but also increase the surface area of the catalyst. Both photocatalytic performance and adsorptivity of the catalyst have been improved. The ternary photocatalyst exhibited over 93 % removal efficiency of typical herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) under simulated solar light irradiation with good stability and easy recovery, which justifies the photocatalytic system, a promising application for herbicide or other organic pollutant removal from water.     

Mesoporous titanium dioxide@ zinc oxide–graphene oxide nanocarriers for colon-specific drug delivery

In this project, TiO₂@ZnO nanoparticles core–shell nanostructured and titanium dioxide@ mesoporous zinc oxide–graphene oxide (TiO₂@ZnO–GO) hybrid nanocomposites as controlled targeted drug delivery systems were synthesized by a facile sono-chemical method. We prepared a novel mesoporous and core–shell structure as a drug nanocarrier (NCs) for the loading and pH-responsive characteristics of the chemotherapeutic curcumin. The structure, surface charge, and surface morphology of NCs were studied using with X-ray diffraction, Fourier transform infrared spectroscopy, dynamic light scattering, brunauer–emmett–teller, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The SEM and TEM images of NCs show the uniform hexagonal mesoporous morphology with average grain size of about ～ 190 nm. The drug loading was very high about 16 and 19 for TiO₂@ZnO and TiO₂@ZnO–GO, respectively. The NCs showed pH-dependent drug release behavior. Drug release from TiO₂@ZnO–GO in neutral pH were higher than in acidic medium, due to anionic charge of GO nanosheet. MTT assay results showed that the curcumin-loaded NCs showed significant toxicity due to which cell viability reduced to below 50% at 140 μg/mL concentration, thereby confirming its anticancer effects. The goal of this study is the application of water-dispersed TiO₂@ZnO–GO with pH-dependent release properties for design a new drug delivery carrier.     

Substitutional Carbon‐Modified Anatase TiO2 Decahedral Plates Directly Derived from Titanium Oxalate Crystals via Topotactic Transition

Changing the composition and/or structure of some metal oxides at the atomic level can significantly improve their performance in different applications. Although many strategies have been developed, the introduction of heteroatoms, particularly anions to the internal part of metal oxide particles, is still not adequate. Here, an effective strategy is demonstrated for directly preparing polycrystalline decahedral plates of substitutional carbon‐doped anatase TiO₂ from titanium (IV) oxalate by a thermally induced topotactic transition in an inert atmosphere. Because of the carbon concentration gradient introduced in side of the plates, the carbon‐doped TiO₂ (TiO_2–xC_x) shows an increased visible light absorption and a two orders of magnitude higher electrical conductivity than pure TiO₂. Consequently, it can be used as a photocatalyst and an active material for lithium storage and shows much superior activity in generating hydroxyl radicals under visible light and greatly increased electrical‐specific capacity at high charge–discharge rates. The strategy developed could also be applicable to the atomic‐scale modification of other metal oxides.     

Defect engineering of photocatalysts for solar-driven conversion of CO2 into valuable fuels

Photoreduction of CO₂ into valuable fuels is a clean and sustainable way to mitigate the energy crisis and environmental problems. Factors limiting the efficiency of CO₂ photoreduction include narrow-band light absorption, poor charge carrier separation and transport, and sluggish activation/reaction of CO₂ on the surface of photocatalyst. In recent years, defect engineering of photocatalysts emerges as an effective method to improve their efficiency in the photocatalytic conversion of CO₂ into useful fuels. This review is focused on discussing how structural defects can be used to modulate the electronic structure of the photocatalysts and activate the inert CO₂ molecules. Special emphasis is placed on the important impact of defects on the charge carrier dynamics of the photocatalysts. Our discussions cover a variety of defective semiconductors, including metal oxides, metal sulfides, and two dimensional materials. In addition, the challenges and prospects of defect engineering in photoreduction of CO₂ are also analyzed. This review aims to provide useful information about the fundamental principles of photoreduction of CO₂ and guidance on the design and preparation of defective photocatalysts.     

Ternary non-noble metal zinc-nickel-cobalt carbonate hydroxide cocatalysts toward highly efficient photoelectrochemical water splitting

TiO₂ photoanodes have aroused intensive research interest in photoelectrochemical (PEC) water splitting. However, they still suffer from poor electron-hole separation and sluggish oxygen evolution dynamics, leading to the low photoconversion efficiency and limiting commercial application. Here, we designed and fabricated novel ternary non-noble metal carbonate hydroxide (ZNC-CH) nanosheet cocatalysts and integrated them with TiO₂ nanorod arrays as highly efficient photoanodes of PEC cells. Compared with the pristine TiO₂, the photocurrent of photoanode with the optimal amount of ZNC-CH represents 3.2 times enhancement, and the onset potential is shifted toward the negative potential direction of 62 mV. The remarkable enhancement is attributed to the suppressed carrier recombination and enhanced charge transfer efficiency at the interface of TiO₂, ZNC-CH and electrolyte, which is closely related to the zinc elements modulated intrinsic activity of catalysts. Our results demonstrate that the introduction of multimetallic ZNC-CH cocatalysts onto photoanodes is a promising strategy to improve the PEC efficiency.     

A Nonmetal Plasmonic Z‐Scheme Photocatalyst with UV‐ to NIR‐Driven Photocatalytic Protons Reduction

Ultrabroad‐spectrum absorption and highly efficient generation of available charge carriers are two essential requirements for promising semiconductor‐based photocatalysts, towards achieving the ultimate goal of solar‐to‐fuel conversion. Here, a fascinating nonmetal plasmonic Z‐scheme photocatalyst with the W₁₈O₄₉/g‐C₃N₄ heterostructure is reported, which can effectively harvest photon energies spanning from the UV to the nearinfrared region and simultaneously possesses improved charge‐carrier dynamics to boost the generation of long‐lived active electrons for the photocatalytic reduction of protons into H₂. By combining with theoretical simulations, a unique synergistic photocatalysis effect between the semiconductive Z‐scheme charge‐carrier separation and metal‐like localized‐surface‐plasmon‐resonance‐induced “hot electrons” injection process is demonstrated within this binary heterostructure.     

Metal Oxide Hollow Nanostructures for Lithium‐ion Batteries

Metal oxide hollow structures have received great attention because of their many promising applications in a wide range of fields. As electrode materials for lithium‐ion batteries (LIBs), metal oxide hollow structures provide high specific capacity, superior rate capability, and improved cycling performance. In this Research News, we summarize the recent research activities in the synthesis of metal oxide hollow nanostructures with controlled shape, size, composition, and structural complexity, as well as their applications in LIBs. By focusing on hollow structures of some binary metal oxides (such as SnO₂, TiO₂, Fe₂O₃, Co₃O₄) and complex metal oxides, we seek to provide some rational understanding on the effect of nanostructure engineering on the electrochemical performance of the active materials. It is thus anticipated that this article will shed some light on the development of advanced electrode materials for next‐generation LIBs.     

The Effect of Materials Architecture in TiO2/MOF Composites on CO2 Photoreduction and Charge Transfer

CO₂ photoreduction to C₁/C₁₊ energized molecules is a key reaction of solar fuel technologies. Building heterojunctions can enhance photocatalysts performance, by facilitating charge transfer between two heterojunction phases. The material parameters that control this charge transfer remain unclear. Here, it is hypothesized that governing factors for CO₂ photoreduction in gas phase are: i) a large porosity to accumulate CO₂ molecules close to catalytic sites and ii) a high number of “points of contact” between the heterojunction components to enhance charge transfer. The former requirement can be met by using porous materials; the latter requirement by controlling the morphology of the heterojunction components. Hence, composites of titanium oxide or titanate and metal–organic framework (MOF), a highly porous material, are built. TiO₂ or titanate nanofibers are synthesized and MOF particles are grown on the fibers. All composites produce CO under UV–vis light, using H₂ as reducing agent. They are more active than their component materials, e.g., ≈9 times more active than titanate. The controlled composites morphology is confirmed and transient absorption spectroscopy highlights charge transfer between the composite components. It is demonstrated that electrons transfer from TiO₂ into the MOF, and holes from the MOF into TiO₂, as the MOF induces band bending in TiO₂.     

Nanoconfined Nickel@Carbon Core–Shell Cocatalyst Promoting Highly Efficient Visible‐Light Photocatalytic H2 Production

The realization of large‐scale solar hydrogen (H₂) production relies on the development of high‐performance and low‐cost photocatalysts driven by sunlight. Recently, cocatalysts have demonstrated immense potential in enhancing the activity and stability of photocatalysts. Hence, the rational design of highly active and inexpensive cocatalysts is of great significance. Here, a facile method is reported to synthesize Ni@C core–shell nanoparticles as a highly active cocatalyst. After merging Ni@C cocatalyst with CdS nanorod (NR), a tremendously enhanced visible‐light photocatalytic H₂‐production performance of 76.1 mmol g^?1 h^?1 is achieved, accompanied with an outstanding quantum efficiency of 31.2% at 420 nm. The state‐of‐art characterizations (e.g., synchrotron‐based X‐ray absorption near edge structure) and theoretical calculations strongly support the presence of pronounced nanoconfinement effect in Ni@C core–shell nanoparticles, which leads to controlled Ni core size, intimate interfacial contact and rapid charge transfer, optimized electronic structure, and protection against chemical corrosion. Hence, the combination of nanoconfined Ni@C with CdS nanorod leads to significantly improved photocatalytic activity and stability. This work not only for the first time demonstrates the great potential of using highly active and inexpensive Ni@C core–shell structure to replace expensive Pt in photocatalysis but also opens new avenues for synthesizing cocatalyst/photocatalyst hybridized systems with excellent performance by introducing nanoconfinement effect.     

Photodecomposition effects of graphene oxide coated on TiO2 thin film prepared by electron-beam evaporation method

Morphological, structural and photocatalytic properties of graphene oxide (GO)/TiO₂ thin-film deposited on quartz substrate were investigated. The TiO₂ film was prepared by electron-beam evaporation and the GO film by spin coating method. The photocatalytic activities of the GO/TiO₂ film were evaluated by photodecomposition of methylene blue. There was synergistic effect between TiO₂ and GO which causes a rapid photo-induced charge separation and the reduction of the recombination of electron-hole pairs under the UV-visible light irradiation. GO on TiO₂ film also promotes the properties of adsorption of the dye, photon scattering probability, and interacting surface area. As a result, it leads to the enhancement of the efficiency of the photodegradation in GO/TiO₂ film.